Wave length control of wave energy



3 Sheets-Sheet 1 Aug. 30, 1949. "R" L. HOLLINGSWORTH WAVE LENGTH CONTROLOF WAVE ENERGY- Filed Jan. 11, 1943 WAVE LENGTH MODULATED STHGIEMODULATION CARRIER SOURCE 6 H R T MEM R AWN M ML 5 R /P E M M Y HE mu mW M T E T LW V T U L 0 N A R .MR E E E D Y RB P FOM u Em .D FT E 1 w U hMM A n1 w (mg Aug. 30, 1949. "R" L. HOLLINGSWORTH 2,480,820

WAVE LENGTH CONTROL OF WAVE ENERGY Filed Jan. 11, 1943 3 Sheets-Sheet 2SHIELD 3'9 7'0 INDICATOR I I 'INVENTOR I 4 7? LEE HOLLINGSWORTH MMATTORNEY Aug. 30, 1949. "R" HOLLINGSWORTH 2,480,820

WAVE LENGTH CONTROL OF WAVE ENERGY Filed Jan. 11, 1943 3 Sheets-Sheet '3U Q 4.5 46 X Y FREQUENCY I 4.9" 4? k 3 r FREQUENCY MODULA T/ON INPUT g5' A- F. OUTPUT '70 60 TO COMPL E X gES/STA/VCE I? OSCILLATOR ,2,

ATIIORNEY Patented Aug. 30, 1949 WAVE LENGTH CONTROL OF WAVE ENERGY "RLee Hollingsworth, Riverhead, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application January 11, 1943, SerialNo. 471,946

13 Claims. 1

This application discloses a new and improved method and system offrequency modulation as applied to radio transmitting and receivingequipment.

In the present invention, I frequency modulate an oscillator by varyingthe electron stream capacity of a vacuum tube to change the capacity ofan associated oscillator tube, or a component part of an oscillatorcircuit.

In another well known system of frequency modulation it has been thepractice to phase modulate the oscillator of a frequency modulationtransmitter in accordance with modified modulation currents, and withseveral frequency multiplying stages of amplification finally arrive atthe assigned carrier frequency.

Yet in another method of frequency modulation, reactance tubes are usedin association with the tank circuit of an oscillator to more directlyproduce frequency modulated waves.

It is, therefore, one of the purposes of this invention to producefrequency modulated electromagnetic waves.

It is another purpose of this invention to render an amplitudemodulation electro-magnetic wave receiver susceptible to frequencymodulated electro-magnetic waves.

Still another purpose of this invention is to produce linear powerconversion from an oscillator using only a small amount of controlvoltage, utilizing a suitable discriminator circuit in association witha suitable rectifier.

Yet another purpose of this invention is to limit automatically themodulation bandwidth of a frequency modulated transmitter.

Further, it is another purpose of this invention to frequency modulate aquartz crystal oscillator.

Also, it is a purpose of this invention to produce improved systems ofuni-directional or single sideband frequency modulation.

It is another purpose of this invention to provide'a capacity composedof the individual capacities between the electrons within an electronstream or cloud, and which is variable as a function of an appliedcontrol voltage.

A prime purpose of this invention is simplification of the systemwhereby those schooled in the art and science that it invokes, caneasily assemble and use the same.

In describing my invention hereinbefore and hereinafter the term"frequency modulation has been used. It is, of course, understood thatthe modulation carried out in my system may be termed wave lengthmodulation and may have the characteristics of frequency modulationprimarily or phase modulation primarily or may have modified forms ofeither thereof or characteristics of both phase and frequency modulationand modified forms thereof. Where the modulation or control is as tophase the modulating potentials are modified in amplitude in accordancewith their frequency as disclosed in Crosby U. S. Patent No. 2,279,659,dated April 14, 1942.

In a thermionic vacuum tube having a cathode or electron emitter,hereinafter called a cathode, and a control grid for controlling theflow of electrons to a plate or electron collector hereinafter called aplate, the electrical capacity between these elements is somewhatdifferent when the cathode is active. With all elements inactive such aswhen the cathode is not emitting electrons, and when there is no controlvoltage applied to the grid and likewise no voltage applied to theplate, the electrical capacity between these elements is constant. Whenthe cathode is active and a stream of electrons is passing from thecathode to the plate, and the grid is tapped into or across this streamof electrons, the capacity between the various elements is somewhatdifferent, and this capacity is variable with the mutual conductance ofthe vacuum tube. This can be explained by assuming that the hot capacityof a vacuum tube consists of the sum of the individual capacitiesbetween each electron and all other electrons within the electron streamor cloud. As small as these electrons are, their capacity may berelatively large in comparison to their surface area because of theirclose association. It is an established fact that the mutual 7 latercircuit capable of being frequency modulated by an associated vacuumtube.

Figures 2a and 2b are modifications of the arrangement of Figure 2.

Figure 3 shows a similar embodiment for frequency modulating a quartzcontrolled oscillator.

Figure 4 illustrates how a p w r os llat r may be similarly frequencymodulated by a small as to conductance by potentials applied at [I inphase with the control of It.

To synchronously tune an antenna to produce uniform radiation over awide frequency band, the cathode of a vacuum tube I6; is connected orcoupled thereto as shown in Figure 2a. The plate of said tube isconnected to ground or to a counterpoise system. The modulating voltageis applied to the grid of the vacuum tube, in the same phase relation asthat applied to the oscillator and amplifier, that is, if the frequencyof the oscillator is increased, the amplifier and antemia would belikewise tuned to a higher frequency in synchronism with the carriershift due to modulation.

A vacuum tube may be connected across a tuned circuit of any desiredvariety, and the electron stream capacity of the vacuum tube varied as aresult of applied grid voltage variations to change the naturalfrequency or resonant point of the tuned circuit in the ether spectrum.

It will be appreciated that a vacuum tube in like manner can beconnected across or in parallel to a quartz plate that is used toproduce stabilized oscillations in association with a vacuum tube in aconventional oscillator circuit to frequency modulate the quartz plateoscillator. Such a circuit is shown in Figure 3, where the oscillatortube is shown at H. The tuned plate circuit is shown at I3 and theoutput circuit is shown at M. The quartz plate 2|, shown in aconventional manner, is connected between the grid and cathode. Themodulating tube I 6' is connected in parallel to the quartz plate. Themodulating voltage is applied to the grid of vacuum tube It to producecapacity variations in parallel to the quartz plate, which varies thenatural operating frequency of said quartz plate in proportion to theelectron stream capacity change within the modulating tube.

Since the power of quartz oscillators is usually low, the output wouldusually feed into another amplifier before being connected to anantenna. The frequency of the quartz oscillator may be multiplied ifdesired.

In Figure 4 a power conversion circuit is shown. Vacuum tube H is shownconnected in an oscillator circuit similar to that shown in Figure 2.Here modulating tube l 6' is shown to have a positive voltage applied tothe plate from battery 24 to increase the electron stream capacitybetween the cathode and plate. The increased grid bias is negligiblesince it may be counteracted by the modulating voltage connected atterminals l1 and I8. Modulating tubes 22 and 23 in shunt respectively totuned circuits l2 and I3 serve to illustrate that additional modulatingtubes may be connected in any part of an oscillator circuit to use theirelectron stream capacity for varying the frequency of the oscillator toa greater degree if so desired. Such a tube may also be connectedbetweenthe grid and plate of the oscillator tube ll, thereby adding thecathode-to-plate capacity of the modulating tube to that of thegrid-to-plate capacity of the oscillator tube. The modulating voltage isapplied to the grid of the modulating tube or tubes as heretoforeexplained.

The action of the circuit of Figure 4 is as follows: The circuit isassumed to be generating a desired amount of radio frequency power.Modulating voltage at a desired or variable frequency is applied toterminals I1 and i8. Vacuum tubes i6, 22 and 23 add or subtract electronstream capacity to or from the oscillator circuit in proportion to theapplied modulating voltage. Circuit 26 is coupled to circuit l3 and istuned to resonance with circuit l3, causing rectifier tube 25 to passmaximum current through resistance 21 which is filtered by condenser 28.The resulting power output appears across terminals 29. Condenser 36 isa radio frequency bypass condenser to prevent transfer of radiofrequency energy from the oscillator past the shield 3i in which theoscillator is housed,

This circuit, Figure 4, may operate to deliver maximum power acrossterminals 29 with no modulating voltage applied, and act to give reducedpower across terminals 29 when the modulation is applied, or theoscillator may be detuned to a point where essentially no power wouldappear across terminals 29, and with modulation applied it would deliverfull or proportionate power (at terminals 29) comparable to the appliedmodulating voltage. The latter method of operation would save power,since the oscillator circuit would draw the maximum power from the powersupply source only during the time that the modulation tunes it tomaximum output. In the language of the art, zero modulation level wouldrequire a minimum of power to be drawn by the oscillator from the powersource, and modulation would cause the oscillator to draw the maximumamount of power from the power source.

A commonly known push-pull type of oscillator may be frequency modulatedin the manner heretofore explained. and in fact, any type of oscillatorcan in this manner be frequency modulated, whose frequency can bechanged by adding or subtracting capacity to or from any component partof the oscillator in question, without departing from the scope of thisinvention.

Further, a coupled circuit in association with an oscillator may beshunted by a vacuum tube as heretofore described, and the electronstream capacity of the vacuum tube varied to produce inductive orcapacitive loading that would change the frequency of the oscillator,thus producing frequency modulation of said oscillator, withoutdeparting from the scope of this invention.

In Figure 5 the same principle is employed to modulate the conventionaloscillator shown. In this embodiment another method of producing singlesideband transmission is shown, together with a means for automaticallycontrolling the modulation bandwidth, and a means for visuallyindicating full modulation. The modulator tube 33 is shown to be acommonly known screen grid type. The cathode of tube 33 is connectedacross the grid tuning circuit l2 to ground. Modulation is accomplishedas heretofore shown by applying the modulating voltage to terminals I!and E8. The plate of the modulator tube is connected through theinductance of tuned circuit 34, which is coupled to circuit I3, andthrough solenoid switch winding 35 to ground. Tuned circuit 36 iscoupled to circuit l3 and connected to the plate of rectifier tube 31.When the rectifier draws current, the path of said current is throughthe inductance winding of tuned circuit 36, resistance 38 and throughthe winding of solenoid switch 39.

The action of this circuit shown in Figure 5 can best be described withreference also to Figure 6. Let us assume that the oscillator is inoperation and that its position in the frequency spectrum is shown to bethat of resonance curve 45, and the oscillator is assumed to beunmodulated. Further, let us now assume that the positive half-cycle ofmodulation is applied at tere tion produced by modulating tube Hi, wecan assume that the receiver would receive amplitude wave modulations ina normal manner. Now, if inaudible frequency wave energy is appliedafter preferably being rectified to terminals l1 and [8, the frequencyof the oscillator will be tuned to one side at the rate of the appliedinaudible frequency, producing a discriminating effect to allow thedetection of frequency modulated waves as the oscillator frequency isvaried in and out of tune with the associated tuned circuits of thereceiver. The radio frequency amplifiers ahead of the conversionoscillator circuit of such a superheterodyne receiver, and theintermediate frequency amplifiers after the conversion oscillatorcircuits, and the associated discriminator circuit may in the samemanner as heretofore eX- plained be tuned and detuned in step with theswing of the oscillator to cause the receiver to be susceptible to broadsweep or wide band frequency modulated waves. The discriminator circuitmay be adjusted to be delivering zero output until the same or separatehigh frequency. modulation is applied to frequency modulate its tuningin a manner to cause it to deliver maximum power at a point in thefrequency spectrum Where the transmitters modulation reached 100%. Sucha discriminator circuit would not be unlike that shown by circuit 26 ofFigure 4, though it would be shunted by the modulating tube such asheretofore shown and explained.

Thus, by throwing a switch to energize an associated oscillator, anysuperheterodyne radio Wave amplitude modulation receiver may become anefiicient receiver of frequency modulated radio waves.

Another application of this principle in a radio receiving system is toproduce fiat top characteristics in radio wave amplifiers, especially inintermediate frequency amplifiers such as are used in superheterodynereceivers. This is accomplished by rapidly tuning and detuning thestages of amplification to cause the maximum response to traverse agiven frequency band in the frequency spectrum.

Further, an oscillator used in checking the characteristics ofamplifiers can be frequency modulated in the manner shown in thisinvention, in the place of using the commonly known motor drivencondenser for varying the frequency While visual characteristics arebeing observed during final adjustments.

Further reference to Figure 5 will show that there is inherentlyincorporated in this circuit a means for preventing a radio receiverfrom drifting away from the desired station being received. Assume thatthe oscillator is that of a conversion oscillator of a superheterodyneamplitude modulation receiver, and further assume that tuned circuits 34and 36 are tuned immediately above and below the desired frequencychannel to be received. If the oscillator drifted higher in frequencythan the desired channel, the energy induced into circuit 34 wouldincrease the plate current through tube 33 to cause a capacity increasethat would again lower the frequency of the oscillator to the desiredfrequency. If the oscillator drifted lower in frequency, circuit 36would produce rectifier current to increase the negative voltage on thescreen grid of the modulator tube to decrease the electron streamcapacity of this tube, thus causing the capacity of the oscillator to belowered, increasing the frequency of the oscillator to the normallydesired channel. It may be assumed that the oscillator 10 is unusuallypowerful and that only a part of its output is being used by thereceiver for frequency conversion purposes, or that tuned circuits 34and 36 are located at the output end of the intermediate frequencyamplifier, where a suitable amount of signal energy is available.

Thus is shown a means for holding an oscillator within a given frequencyband which may be used in either radio receivers or transmitters, orwherever a controlled oscillator can be used to advantage.

It will be further appreciated that this type of frequency modulation isessentially basic in nature, and that there are many possible uses forsuch a variable electron stream or electron cloud capacity, such as, forinstance, to displace the directivity of a directive antenna to causethe beam to cover a wider area, such displacement to take place at avariable or voice frequency, or at a constant rate of variation such aswould be used in radio telegraph communication to cover effectively awider area, and to reduce fading somewhat at the higher frequencies suchas are used for long distance communication.

It will be still further appreciated that all of the elements of tubesl6 and Il may be housed in the same bulb or container if so desired.

The applications of this type of variable capacity seem to be sonumerous, that in the claims to follow, one basic claim thereforeappears to protect this invention in the broad field that itencompasses. An application of the principle chiefly involved in thisinvention is employed in my United States Patent No. 2,243,423.

What is claimed is:

1. In a wave generating and wave frequency control, an electron streamcapacity in a tube comprising a heated cathode, a plate, and at leastone control grid, said electron stream flowing from the cathode to theplate, a generator tube having a grid and a cathode, a conductiveconnection connecting said cathode of the first tube to the grid of thegenerator tube, a conductive connection between the plate of saidelectron stream capacity tube and the cathode of said generator tube,and connections for applying a control potential to the grid of saidelectron stream capacity tube to vary the electrical capacity of theelectron stream to control the frequency of the oscillations generatedby said generator tube.

2. In a wave generating and wave frequency modulating system, anoscillation generator including an electron discharge device having aselectrodes an anode, a cathode and control grid and including generatingcircuits including a quartz crystal coupled with the device electrodes,an electron stream capacity in a tube comprising a heated cathode, aplate, and a control grid, said electron stream flowing from the cathodeto the plate, a substantially direct connection between the cathode ofsaid tube and the grid of said device, a substantially direct connectionbetween the anode of said tube and the cathode of said device, andconnections for applying a controlling potential to the grid of saidelectron stream capacity tube to vary the electrical capacity thereofand in turn modulate the frequency of the oscillations generated in saiddevice as controlled by said quartz crystal.

3. In a power conversion system, an oscillation generator comprising adischarge ,device having a control grid electrode and cathode electrodeand having output electrodes, means coupling said two first-namedelectrodes in an oscilgrease-c l in i li w jin l i a't oiltputjcircuit atuning' reactance coupIl g:saidbu putcircuitto'said output electrodes,amcamat rrti se having a' cathode coupled to theficontroljgridof saiddevice and having an anodejcoupled to" the cathode of said device, saidrnodulat or tube having a control grid, a secondmodulator tube having acathode and an anofdrej coupljedfljwith said'tuning reactance, saidsecondmodulator tube having a control grid, and meansjor applying acontrol potential to be conve djsimhltaneously to'the control grids ofsaid'modulat rtu e [51} lnfjaisystem of the class described, an oscil-Iationjgenerator comprising a tube having a control'jgri'dand a cathode,an electron stream capacity comprising an electron discharge devicehaving ,a'catl o'degja' plate and a control grid ele ode; said'electronstream flowing irom the cathodej'td the plate of said device, means for.ctnneenng the cathode of said device to the cbntrol grid of said tube,a connection between the plate of s'aid device andthe cathode of saidtube; and connections for applying a pulsating negative electromotiveforce to the grid of said lcCtiolLStrfea-in" capacity device to reducethe eI'ectricaLcaigacity or the electron stream to therebyincre'ase thefrequency of the oscillations generated, thufsproducing uni-directionalor singlesidebandwave length modulation.

5.1m a'systemofgthejclass described, an oscillati'o'nj gen ratorcomprising a tube having a controlfglid, and afcatho'de, an electronstream capacity comprising an electron discharge device having acathode, a'plate and a control grid electrode, said electron streamflowing from the cathode tothe plate of said device, means forconnecting the cathodeof said device to the con- .trolgrid of,jsaidtube, a connection between the plate of said device and the cathode ofsaid tube, and, connections ror applying a pulsating positiveelectromotive forceto the grid of said electron streamv capacity deviceto increase the electrical capacity of the" electron stream to therebydecrease the ,frequnf yoi the oscillations generated, thusproducingunidirectional or single sideband wave length modulation.

In a] system of the nature described, an electron, discharge tube havinginput and output electrods incliidiiig a control grid, an anode, and 'a'cathode. with 'a tuned circuit coupled between the jgrid,,,ar ijdcathode, an electron stream capacitytcoinprising an electron dischargedevice having aheatedcathode, a plate, and a control grid, the electronstream in said device "flowing from the cathode to the plate, a con-:ductive connection between the cathode of said device and control gridof said tube, a connection between the anode of said device and the,Icatho'de of saidv tube, and means for connecting a,pulsatingoralternating current source to the control grid of sai'ddevice to vary the electrical ca acity of the electron stream to therebyvary the tuning of said tuned circuit. 7.,In a systemoithe classdescribed, a plurality, of electron discharge devices coupled in,cascade by a plurality of tuned circuits with means ,for setting .uposcillations in the first of said stages and deriving amplifiedoscillations from the last of said stages, a plurality of electronstream capacity tubes each having a heated cathode, a plate anda controlgrid with the elec- ,,tfoI 1' stream flowing from the cathode to theplate, therebeinga tube for each of the tuned circuits it is' "desiredto control, connections bevery close to their 12 tw fithe catnbdesci'eacii er safe tubes-and one side ofadiifeieiit one-bi-said-tun'ed'circuits, connebtiqngj bet een the anod'es of-each' of saidtubes and-the "othei 'sidewf a different one of said tuned 'cirjcijit'athle arrangement being such that the'impedahcbtwecn' the anode andcathode of each of saidtubesis connected across a differentassassin-"tunedcircuits, at source of control potential;and 'connecticnsfor applying control 'potefitial' from said source to the control gridsof each of said tubesin phase synchronism.

8, In an arrangenifnt'of theclass-described, a wave generator-devicehavinga grid and a "oathode, an electron stream capacity produced in athermionic"vacuum fiubeconsisting of a heated cathode; a plate; andat-"leas't two control grids, saidfelectrori-stiahi flowing from 'the'cathode to the plate 'to comp'risaa' wave length modulator tube, aconnectio'n-lietWeen-the cathode of the tube and thegii'dbfsaid device,a connection betwee'n th'e plate of; said'electron stream capacitytub'and the cathode of the device, said connection including a 5 solenoidswitch winding and aninductance of a first tunedradio frequencycircuitatt n'e'd to th'' upper'side of the generatorsf dulatedireqilency', a, second tunedradiojreduehcyf cii'cl'i'itcoupled to thewave v generator ascertained to the lower side of the geiielators umeasured frequency and connected in series with a resistanceand 'a'solenoid inging a a rdrtnercenneeiee' to the plate and-cathode remeansfora i plying an electromotive or firstcontrolgrid of the wereby'th'e fi T L second of 'tuii circuits become energized whenfull'mo: la Ii iS aEcompliShed to either increase or de H V tes ament ofthe electron stream'vvit them'odulat'ing tube in opposition to the capacty'changeasaresult of the"electro motive force afiplied to" th firstcontrol grid of ime. ;.neee=Sca n s h wave lengths of w hich 'are tojbe;controlled, flow, said circuit includih controllable reactancecomprising an elec ron st earn capacity produced in a thermionic"vacuui'ri tub e, connected to said tuned circuit towave length modulatethe oscillations, s'aid tube having two grids, means for applying analte ng current electromotive force to one grid tube to'controlitscapacity, a second I une adio frequency circuit coupled tothc first't'ed circuit andattuned er'icyof the oscillations when not mbdulate ndmeans for applying the energy induced in dLSlion'd tuned circuit toanother g' ridof the le res stream capacity tube to eliminate the an onoii'one sideof the unmodulated 'walehgth when modulation is g Vsectional or single sid'eband waive-lengthjnodul ation. 7

10. In an arrangemeht of the; class described,

a tun'ed circuit wherein wave energy to be modulated flows, saidciruitinclu'cling an electron stream capacity-produced in'a thermionicvacuum tube having a cathcide and at'lea st two other electrodesincluding an -sameand cathode, c'onnecti'o'n's coupling'th' anode andthe cathode of said electron' stream capacity tube to said tunedcircuit, two closely assdi'atedtund circuits attuned ahove and'filoivthe desired operating frequency of said first tuned circuit and coupledthereto, and circuits for applying the currents from the respectivetuned circuits to said two other electrodes to produce a capacity changeto oppositely oppose frequency drift of the wave energy in said tunedcircuit.

11. In a system of high deviation frequency modulation transmission, amaster oscillator, means for modulating the master oscillator includingan electron stream capacity vacuum tube, means for controlling thebandwidth of the modulation applied to the master oscillator, afrequency multiplier-amplifying system coupled to the oscillator, meansfor tuning said frequency multiplier-amplifying system in synchronismwith the modulation applied to said master osciliator, an antenna systemcoupled to said frequency multiplier and a means for tuning the same insynchronism with the modulation of the master oscillator and with thesynchronous tuning of the associated frequency multiplier-amplifyingsystem.

12. In signalling apparatus, means for producing wave energy comprisingan electron discharge device having an anode, a cathode, and a controlgrid, with wave energy producing circuits connected thereto, an electronstream capacity in the form of a thermionic vacuum tube having a heatedcathode, a control grid and an anode, the capacity being formed by theelectron stream between the cathode and anode, a connection, of lowimpedance to voltages of the frequency of the wave energy produced,between the anode of said tube and the cathode of said device, aconnection, of low impedance to voltages of the frequency of the waveenergy produced, between the cathode of said tube and the control gridof said device, and connections to the control grid and cathode of saidtube for varying the potential on the control grid relative to thecathode to modulate the frequency of the wave energy produced in saiddevice, the arrangement being such that the Wave energy may be modulateda portion of a cycle thereof or through several megacycles.

13. In a signalling system, a first alternating current circuit tuned toa selected frequency, means for setting up alternating current waveenergy in said circuit of a frequency determined by the tuning thereof,an electron stream capacity in a tube having electrodes including a I4heated cathode, an anode, and at least one control grid, the capacitybeing formed between the anode and cathode of said tube, connectionsconnecting the anode and cathode impedance of said tube in shunt to saidtuned alternating current circuit, a connection to the control grid ofsaid tube for applying a control potential thereto for varying theelectron stream between the anode and cathode of said tube to therebyvary the capacity of the tube and the tuning of said alternating currentcircuit and the frequency of the wave energy therein, a secondalternating current circuit excited by current set up in said firstcircuit and tuned to a frequency above said selected frequency, meansconnected with said second tuned circuit for rectifying current set uptherein for producing a potential and applying the same to an electrodein said tube to control the capacity thereof when the frequency of thealternating current in said first circuit attains the frequency to whichsaid second circuit is tuned, a third circuit excited by current set upin said first circuit and tuned to a frequency below said selectedfrequency, means connected with said third circuit for rectifyingcurrent set up therein for producing a potential and applying the sameto an electrode in said tube to control the capacity thereof when thefrequency of the alternating current in said first circuit attains thefrequency to which said third circuit is tuned, and an output circuitcoupled to said first mentioned alternating current circuit.

R" LEE HOLLINGSWORTH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,628,905 Nyman May 17, 19271,716,161 Allcutt June 4, 1929 1,777,410 Jones Oct. 7, 1930 2,032,620Langmuir Mar. 3, 1936 2,121,737 Hansell June 21, 1938 2,217,417 PetersonOct. 8, 1940 2,243,829 Brett et al. June 3, 1941 2,274,184 Bach Feb. 24,1942 2,274,648 Bach Mar. 3, 1942 2,282,103 Tunick May 5, 1942 2,337,214Tunick Dec. 21, 1943

